A Global System of Mobile Communication (GSM) or Universal Mobile Telecommunication System (UMTS) or Evolved Packet System (EPS) mobile device tries to select and register to certain networks in priority order based on certain user and operator settings as specified in 3GPP TS 22.011 (Services and System Aspects; Service Accessibility) and 3GPP TS 23.122 (Core Networks and Terminals; Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode).
Only mobile devices with valid Subscriber Identity Module (SIM) or Universal Subscriber Identity Module (USIM) inserted are allowed to attempt a registration on the network for Circuit Switched (CS) or Packet Switched (PS) services. Mobile devices which are successfully registered on a network for Packet Switched (PS) services may also attempt to register on an Internet Protocol Multimedia Subsystem (IMS) if available and if equipped with a Universal Subscriber Identity Module (USIM) or with an IMS Subscriber Identity Module (ISIM).
Once registered on a network, the mobile device displays the name of the network as specified in 3GPP TS 22.101 (Services and System Aspects; Service aspect; Service principles). In networks where the name is not broadcast it can be derived from the Mobile Country Code (MCC) and the Mobile Network Code (MNC) which are always broadcast in a radio cell of the network. The numerical value of the Mobile Country Code (MCC) and the Mobile Network Code (MNC) is translated into a name by using a list which is stored in the non-volatile memory of the mobile device or additionally on the Subscriber Identity Module (SIM) or the Universal Subscriber Identity Module (USIM).
If all registration attempts fail, e.g. because they were rejected by the available networks or registration is not possible due to a missing Subscriber Identity Module (SIM) or a missing Universal Subscriber Identity Module (USIM), the mobile device will go to the limited service state, i.e. it selects a certain network based on radio conditions without trying to register on that network.
Under certain conditions, a mobile device might register on a network for Packet Switched (PS) services (e.g. with the Subscriber Identity Module (SIM)) but not being able to register on the Internet Protocol Multimedia Subsystem (IMS) (e.g. due to a missing Universal Subscriber Identity Module (USIM) or a missing IP Multimedia Services Identity Module (ISIM)), which would be necessary to setup emergency calls via the Packet Switched (PS) system.
Emergency calls may be attempted by unregistered mobile devices. Dependent on local regulation there are three different possible scenarios:
all unauthenticated emergency calls are allowed, both in the case where a Subscriber Identity Module (SIM) or a Universal Subscriber Identity Module (USIM) or an IP Multimedia Services Identity Module (ISIM) is not present or invalid and when registration is rejected by the network, e.g. due to the lack of roaming agreement;
unauthenticated emergency calls are only allowed if the mobile device has a Subscriber Identity Module (SIM) or a Universal Subscriber Identity Module (USIM) or a IP Multimedia Services Identity Module (ISIM) inserted;
unauthenticated emergency calls are not allowed at all.
These scenarios are applicable for emergency calls in the Circuit Switched (CS) domain as well as for emergency calls in the Internet Protocol Multimedia Subsystem (IMS) via the Packet Switched (PS) domain.
As discussed, when the mobile device is not successfully registered on the network, it enters the limited service state and the 3GPP specifications prescribe that in this state only emergency call attempts are allowed. Depending on the implementation, the mobile device may display in limited service state either “no service” or a message indicating that only emergency calls are allowed (e.g. “emergency calls only”, “SOS”). However this display does not take into account the actual network configuration and regulatory regime of the country where the mobile device is. Therefore, users might try to setup emergency calls repeatedly although it is forbidden or they might not try to do so although it would be possible.
An Internet Protocol Multimedia Subsystem (IMS) capable mobile device being registered on a network for Packet Switched (PS) services only (but not on the Internet Protocol Multimedia Subsystem (IMS)) might just display the network name. However, unauthenticated Internet Protocol Multimedia Subsystem (IMS) emergency calls may not be supported and again the user might be misled to think that emergency calls are supported whereas they are actually not.
Furthermore, as the only criteria for camping on a certain network without registration are radio conditions, the situation may occur in border areas that the mobile device is camping on a network which prohibits unauthenticated emergency calls whereby a different network which would allow those calls would be available as well.